Cushioning systems used in athletic footwear are designed to be capable of attenuating a wide range of impact force magnitudes. Ordinary impact forces in walking and running, for example, vary between approximately 600 Newtons (N) and 2500 N. However, values as high as 15,000 N have been measured as a consequence of certain extreme maneuvers, for example, in the sport of skateboarding. (See: “Impact Forces During Skateboarding Landings,” J. Determan, et al., Proceedings, Thirteenth Biennial Conference, Canadian Society for Biomechanics, Halifax, Aug. 4-7, 2004, page 28). Because the magnitudes of these forces are dependent on body mass, for convenience, impact force data is often normalized to body weight ((body mass)×(acceleration due to gravity)) and described as multiples of body weight. In this manner, these impact forces can be described as varying between approximately 1 Body Weight (BW) and up to and exceeding some 20 BW, in extreme cases.
Because of the wide range of impact forces that athletes experience while practicing their sport, particularly forces involved in high-impact or extreme sports such as skateboarding, no single conventional shock absorption system will satisfy all of athletes' needs. Ordinary impact forces, which may range from 1 BW to 5 BW, such as those experienced in walking, running, and other non-extreme sports, are also encountered in extreme sports, such as skateboarding. The majority of impact forces that skateboarders encounter, for example, are in the range from approximately 1 BW to 5 BW. However, oftentimes during a typical day of skateboarding, extreme impacts on the order of 6 BW to more than 15 BW may be generated in attempting and performing maneuvers that involve large vertical displacements.
Shock attenuating systems that address moderate, ordinary impacts are generally not suitable for extreme impacts due to limitations on physical properties of common shock attenuating systems. For example, one common shortcoming is that these systems reach their displacement limit or “bottom out.”
One common type of material used in athletic shoe shock attenuating systems, polymeric foams, receive their shock attenuating properties principally from the many small gas bubbles trapped in the foam's polymeric matrix. They operate similarly to an inflated shock attenuating system that works by trapping air in a bladder. When a typical polymeric foam, or similar air inflated shock attenuating system, is exposed to high impact forces, the gases within are compressed and reach their displacement limit, thus, becoming non-compliant and ceasing to provide further shock attenuation. The same problem exists for other shock absorbing systems that are more structural in nature, such as springs or molded plastic structures.
Some designs have sought to improve upon the above shortcomings by utilizing a structure that is stiffened or enlarged, or, in the case of foams or inflated systems, the gas volumes and pressures in certain materials have been raised to a high enough level to be able to accommodate higher impact forces. At ordinary levels of impact, however, the resulting systems may often be too thick or too stiff and uncomfortable. Thus, generally speaking, conventional shock-attenuating systems suffer from being useful over only a narrow range of impact forces and tend to have undesirable physical properties when impacted outside that narrow range. Thus, these systems are undesirable for extreme sports, such as skateboarding, where shock attenuation is needed for a very broad range of impact forces.
Shock attenuating systems may be generally described in terms of point-elastic and area-elastic systems. A point-elastic shock attenuating system deforms non-uniformly (see FIG. 1). That is, for example, the greatest compliance is found under the area of highest pressure and the amount of deformation of the shock-attenuating layer varies in proportion to the distribution of forces over its surface. Standing on an inflated air mattress is an example of point-elastic behavior; the area just beneath the foot where pressures are high shows the greatest deformation while other areas show little or no deformation. Meanwhile, an area-elastic system distributes forces over a wider area causing a much greater area of the shock attenuating structure that is engaged in shock attenuating (see FIG. 2). A stiff sheet of plywood laid over an inflated air mattress is an example of an area-elastic system, because the forces applied by standing on the plywood are distributed over a much larger portion of area of the air mattress.
In order to improve upon these conventional shock-attenuating systems, several systems have been developed using combinations of shock absorbing materials in order to provide shock absorption over a broader range of impact forces. U.S. Pat. No. 4,506,460 to Rudy, for example, discloses the use of a stiff moderator to distribute the forces of impact over a larger area of the shock attenuating system. The use of such moderators, however, further restricts the range of impact shocks that can be accommodated because the stiff moderator is limited in its shock absorbing abilities. While successfully distributing forces over a wider area, the stiff moderator fails to adequately absorb high impact forces. Another approach to providing shock attenuation is disclosed by U.S. Pat. No. 4,183,156 to Rudy. Rudy's patent discloses an air cushion for shoe soles that uses a semi-rigid moderator in order to distribute the loads over the air cushion. While moderating the cushioning forces, this system suffers from some of the same shortcomings as that of the area-elastic systems discussed above. Also, the patent fails to disclose a method for providing dynamic moderation of the forces.
Another such spring moderator is disclosed by U.S. Pat. No. 4,486,964 also to Rudy. The '964 patent discloses the use of a moderator having a high modulus of elasticity over a cushioning material. The '964 patent, however, fails to disclose the use of a non-Newtonian material as an improved, dynamic moderator. A cushioning system that utilizes a stiff layer of material sandwiched between two foam, midsole layers is disclosed by U.S. Pat. No. 4,854,057 to Misevich et al. Misevich's patent, however, fails to disclose a cushioning system that uses the advantageous features of both Newtonian and non-Newtonian materials.
Another such system is disclosed by U.S. Pat. No. 5,741,568 also to Rudy. Rudy's '568 patent discloses the use of a fluid filled bladder surrounded by an envelope, in order to combine the properties of compressible padding materials with the effects of fluid materials.
The use of non-Newtonian materials, particularly dilatant materials, has also been used in shock attenuating systems, in order to provide a broader range of impact force attenuation. A non-Newtonian material is a material, often a fluid or gel or gel-like solid, in which the stiffness of the material changes with the applied strain rate. Newtonian materials, meanwhile, are said to behave linearly in response to strain rate so their stiffness is constant over a wide range of strain rates.
Most materials used in shock attenuating systems are somewhat viscoelastic and are not perfectly Newtonian, but the degree to which they are sensitive to the rate of loading is negligible when compared with materials with distinctly non-Newtonian properties.
“Newtonian materials” as we define them for the purposes of this invention, are compliant shock attenuating materials with predominately linear load displacement characteristics. Such Newtonian materials may demonstrate some non-linear properties in imitation of non-Newtonian properties, but they are essentially linear in their load displacement behavior. Furthermore, any distinctly non-Newtonian behavior of these materials can be explained by bottoming out, or, by extreme physical deformation of the material, and not by the fundamental physical and chemical properties that create the character of truly “non-Newtonian materials.”
Materials that qualify for use as Newtonian in an effective cushioning system must be compliant enough to attenuate peak impact forces. Compliance in this context is the strain of an elastic body expressed as a function of the force producing that strain. Compliant shock attenuating systems in footwear are used to decelerate the mass that is producing peak impact forces. These compliant materials yield to the force of impact, but resist with proportional stiffness to decelerate the impacting mass in a controlled manner, thus reducing peak forces, and delaying the time to peak impact. Therefore, an effective Newtonian material must be relatively linear in its load displacement properties, but also compliant enough and thick enough to significantly attenuate peak impact forces. A non-compliant material would not be able to attenuate peak forces, and a material that was compliant, but too thin, would bottom out and be inadequate as a shock attenuating material.
Non-Newtonian properties, meanwhile, are commonly described as either dilatant or pseudo-plastic. Dilatant materials demonstrate significant increases in stiffness as loading rate increases. Pseudo-plastic materials, on the other hand, show the opposite response to increased rates of loading, i.e., their stiffness decreases as loading increases.
U.S. Pat. Nos. 6,701,529, to Rhoades et al. and 5,854,143, to Shuster et al., disclose the use of dilatant materials to moderate the impact forces of a fall or of a ballistic collision. Neither of these patents, however, discloses the use of dilatant materials in combination with a layer of shock absorbing material for attenuating shocks over a broad range of impact forces. What is more, at higher rates of loading and higher force magnitudes, these dilatant materials by themselves would be relatively stiff and non-compliant. Thus, the use of these materials would be undesirable in applications where attenuation of high impact forces is required. Using a dilatant material by itself means that higher impact loads induce an instantaneous increase in stiffness that make the material less shock attenuating. Accordingly, the dilatant material when used by themselves, may be less useful as a shock attenuating material. At the very instant that they need to provide the greatest amount of compliance and shock attenuation, they are less compliant and less shock attenuating.
The device shown and described in U.S. Pat. No. 6,913,802 appears to disclose a dilatant material that is used by itself to attenuate shocks. Foam appears to be attached to the dilatant material but does not appear to serve the purpose of shock attenuation. In support thereof, Col. 4, Lines 5-8 of the '802 application describes the foam as increasing comfort for the wearer.
Another approach to using a combination of materials for shock attenuation is disclosed by U.S. Pat. No. 7,020,988 to Holden et al. Holden's invention discloses a shock attenuating system wherein a system used to attenuate the lower range of impacts is used in combination with a non-compressible second system that is engaged and allowed to provide shock attenuation for the higher range of impacts. Thus, this system allows for both extreme and ordinary impacts to be attenuated. This combined system, however, remains limited by the narrow physical properties of the two individual systems that have been selected for use. Also, the response of the combined system is limited because the two-component system is somewhat discontinuous in its shock attenuating properties.
Thus, there remains a long felt need in the art for a shock attenuating system that is responsive to a broad range of impact force magnitudes, that provides attenuation fairly continuously over a wide range of forces, and that responds to these forces proportionally and adjusts automatically to the actual impact load that it is called upon to absorb.